1. Field of the Invention
The present invention relates to an optical element having a function of suppressing a quantity of light reflected by an interface in an incidence/emission surface for light, a method of manufacturing the optical element, and an optical apparatus using the optical element, and is suitable for an image pickup device such as a camera or a video camera, or an optical scanning device used in a liquid crystal projector or an electrophotographic apparatus (such as a digital copying machine or a laser beam printer).
2. Related Background Art
Generally, in an optical element in which a quantity of light reflected on a surface is required to be suppressed, a signal layer or a plurality of layers of optical films having refractive indices different from one another are laminated on a surface of the optical element by a thickness of several tens to several hundreds of nanometers to obtain desired reflection characteristics. In order to form those optical films, there is used a vacuum film depositing method such as a vapor deposition method or a sputtering method, or a wet film depositing method such as a dip coating method or a spin coating method. Even with any of those film deposition means taken, the film deposition must be carried out after an optical element base is processed, so the optical element is difficult to manufacture, and thus there is a limitation in the reduction of cost.
On the other hand, it is known that the quantity of light reflected by the interface can be suppressed by a fine shape which is formed at a pitch equal to or smaller than a design wavelength on a surface of an optical element without using any of optical films. If the fine shape can be formed in a mold by utilizing that principle and an optical element can be manufactured along with forming of a base, it becomes possible to ultimately reduce the manufacturing cost.
Heretofore, a semiconductor manufacturing process has been widely used as a technique for forming a fine shape called a Sub Wave-length Structure (SWS). With this method, there is an advantage that a precisely designed SWS can be formed, but this method involves a problem in that there are many limitations to a case where the SWS is formed over a large area on a curved surface, and thus the SWS is very difficult to inexpensively (simply) manufacture.
On the other hand, a technique for forming the SWS by utilizing minute particles is proposed as one technique for simply manufacturing the SWS (refer to Japanese Patent Application Laid-Open Nos. 2000-071290 and 2001-074919 for example).
In a case of the technique disclosed in Japanese Patent Application Laid-Open No. 2000-071290 A, when the minute particles are utilized, the SWS can be formed in lump so as to have a large area. However, since the minute particles are continuously and uniformly arranged to construct the SWS, there has been a problem in that it is difficult to control a volume ratio or aspect ratio between a base material and an atmosphere, which determines reflection characteristics, and thus it is difficult to obtain an ideal anti reflection effect.
On the other hand, an anodic oxidation method is known as a technique with which the SWS can be inexpensively formed over a large area, and the aspect ratio can also be arbitrarily controlled. A metal such as aluminum as an anode is anodized in an acid electrolytic solution by supplying a current between the metal having the anode and a cathode, thereby forming fine holes. A technique for regularly arranging holes by utilizing the anodic oxidation, a technique for filling a different material in holes, and the like have been developed (refer to Japanese Patent Application Laid-Open No. H02-254192 and U.S. Pat. No. 6,139,713 for example).
When light is made incident to an optical element, unnecessary reflected light is generated from an incidence/emission surface of the optical element. A problem caused by the reflected light generated from the incidence/emission surface of the optical element at this time will hereinafter be described by giving a conventional laser beam printer (LBP) as an example. However, a problem to be solved by the present invention is related to nonconformity caused by Fresnel reflection generated on an interface to/from which light is made incident/emitted, and thus is not limited to the reflection of light from a surface of a solid.
FIG. 8 is a cross sectional view (main scanning cross sectional view) of a main portion in a main scanning direction of a conventional optical scanning device used in an LBP or the like.
In FIG. 8, divergent light beams emitted from a light source means 91 are made nearly parallel light beams or convergent light beams by a collimator lens 92. The resultant light beams (light quantity) are shaped by an aperture stop 93 to be made incident to a cylindrical lens 94 having a reflecting power only in a sub scanning direction. Of the light beams made incident to the cylindrical lens 94, the light beams within a main scanning cross section are emitted as they are, and the light beams within a sub scanning cross section are converged to be imaged nearly in the form of a line image in the vicinity of a deflecting surface 95a of an optical deflector 95 having a rotating polygon mirror.
Then, the light beams reflected and deflected by the deflecting surface 95a of the optical deflector 95 are guided to a photosensitive drum surface 97 as a surface to be scanned through imaging optical means (fθ lens system) 96 including two fθ lenses 96a and 96b each having fθ characteristics. The photosensitive drum surface 97 is optically scanned in a direction indicated by an arrow B (main scanning direction) with the light beams by rotating the optical deflector 95 in a direction indicated by an arrow A to record image information on the photosensitive drum surface 97.
In recent years, the fθ lens (optical element) constituting imaging optical means is constructed so as to have a free curved surface shape in many cases, and is generally made of plastic material with which the free curved surface shape is easy to make.
However, since an anti reflection film is difficult to be formed on a surface of a plastic lens from the technical and cost reasons, the anti reflection film may be omitted. As a result, the surface reflection may be generated from each optical surface to cause nonconformity. In other words, the surface reflected light generated from the surface of the fθ lens in which an anti reflection film is omitted may be reflected by other optical surfaces to finally reach a non-intentional portion on a surface to be scanned, thereby causing a ghost phenomenon.
In particular, when as shown in FIG. 8, the optical surface (fθ lens surface) 96a1 of the fθ lens of the two fθ lenses which is disposed relatively nearer the optical deflector 95 has a concave shape and each of the incident light beams has a nearly perpendicular incidence angle, the nonconformity may be caused in which after the surface reflected light from the optical surface 96a1 returns back to the optical deflector 95 and is reflected again by a deflecting surface (reflecting surface) 95a of the optical deflector 95 to pass through the imaging optical means 96, the reflected light reaches a non-intentional portion on the photosensitive drum 97 to cause a ghost phenomenon.